The present invention relates to a method and apparatus for cleaning the exhaust gas of an internal combustion engine, and more specifically to a method and apparatus which uses an NOx pickup to determine the amount of oxygen present in an exhaust gas by measuring the decomposition of NH.sub.3 in the NOx pickup.
It is known to use a three-way catalyst in the exhaust gas tract of an internal combustion engine to clean the exhaust gas. It is also known to use provide a lambda probe whose output signal is dependent upon the residual oxygen content in the raw exhaust upstream from the catalyst. It is believed that the residual oxygen content in the raw exhaust depends upon the fuel/air mixture dispersed in the internal combustion engine. It is known that in case of excess fuel (rich mixture), the residual oxygen content in the raw exhaust is lower, and in case of excess air (lean mixture), the residual oxygen content in the raw exhaust is higher.
It is known that in a lean mixture (lambda&gt;1), an output voltage of the lambda probe is usually less than about 100 mV. It is also known that the output voltage of the lambda probe changes almost abruptly near lambda=1, and the output voltage of the lambda probe increase to about 0.8 volt in rich mixture (lambda&lt;1). This is referred to as a two-point action.
There are also known broad-band lambda probes that are capable of outputting signals corresponding to a lambda range (0.7 to 4) in a linear fashion.
It is known to operate an internal combustion engine such that the signal output by the lambda probe, indicating the residual oxygen level in the raw exhaust, varies around a predetermined average which corresponds to a value for lambda=1, for example. It is believed that a three-way catalyst has a certain lambda value .lambda.o with optimal catalytic properties. Thus, the predetermined average of lambda values during the operation of the internal combustion engine should actually be substantially .lambda.o. It is also known, however, to vary the lambda value .lambda.o for optimum catalytic properties from lambda=1, for example lambda=0.99.
It is believed that static and dynamic properties of the lambda probe upstream from the three-way catalyst are altered by aging and poisoning. It is believed that this causes a position of a signal level corresponding to .lambda.o to change. It is known to use an additional lambda probe downstream from the three-way catalyst, which is believed to be less sensitive to poisoning. It is believed that the additional lambda probe may serve as a monitor probe to measure and confirm the catalytic conversion and permit fine regulation of the fuel/air mixture by correcting the signal level corresponding to .lambda.o in the lambda probe upstream, such that the lambda value .lambda.o most favorable to catalytic conversion is maintained. This process is called "guiding control" or "trimming."
It is believed that in order further to reduce pollutant emissions from internal combustion engines, an NOx catalyst can be provided in addition to the three-way catalyst. It is known that the NOx catalyst can also be integrated into the three-way catalyst. It is also known to use a storage catalyst that includes one state of operation which stores NOx and another state of operation which converts the stored NOx. It is known to use a measuring pickup sensitive to NOx downstream from the NOx catalyst to achieve the optimum operation of an NOx catalyst.
The present invention addresses the problem of substantially effectively reducing a residual oxygen content of an exhaust gas of an internal combustion engine without the use of an additional lambda probe downstream of a catalytic converter.